Mid-packet clear channel assessment

ABSTRACT

A mid-packet detection technique is provided that detects a packet with periodic repetitions of a fixed duration at a point in time of the packet other than a start-of-packet pattern, e.g., a preamble, associated with the packet. The process performs packet detection without detecting a preamble and does not require carrier frequency recovery, timing recovery (synchronization) or channel estimation. In one embodiment, a doubly differential matched filter autocorrelation of the received signal is computed and used as a metric for packet detection when the preamble is not observed or to complement preamble detection. The metric is compared to a threshold to indicate detection of a packet.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/202,782, filed Sep. 2, 2008, the entirety of which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure generally relates to communication technology,and in particular to detecting presence of transmissions on acommunication channel.

BACKGROUND

In certain wireless communication systems, multiple devices share accessto the wireless communication medium, e.g., radio frequency (RF) channelor channels. Numerous communication protocols have been developed tocontrol how devices access the medium. One technique that is commonamong many communication protocols is to detect the presence of atransmission from another communication device before a device initiatesits own transmission on the medium. Transmissions are formatted aspackets or frames, where an initial portion of the packet or framecomprises a start-of-packet pattern often referred to as a preamble. Thedata or payload of the packet follows the preamble.

It is common practice to configure communication devices to detect apacket or frame associated with a transmission on the communicationchannel by identifying the preamble associated with the packet. Forexample, received signal strength indicators, changes in energy, andauto- and cross-correlation functions are examples of techniques todetect a preamble. However it is not always possible for a device toobserve the preamble (e.g., due to a collision of one or more packets orshutting down during a power save mode), and in fact the firstobservation that a communication device makes may comprise portions of apacket or frame that follow the preamble (e.g., data symbol) in whichcase the ability to detect a preamble alone is not a sufficient packetdetection technique. In many cases, a communication device fails to seethe transmission of a packet in the absence of detecting the preamble,except when an energy detection threshold is set to a relatively lowvalue, in which case, false positive packet detections may occur.

A technique that allows a communication device to detect an in-progresspacket without requiring detection of a start-of-packet pattern, e.g., apreamble, would be very helpful in improving performance ofcommunication devices and the systems or networks in which thecommunication devices operate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a timing diagram of a packet or frame that isdetected with the mid-packet detection process described herein.

FIG. 2 is an example of a block diagram of a communication device thatis configured to perform the mid-packet detection process.

FIG. 3 is block diagram showing an example of hardware logic that isconfigured to perform the mid-packet detection process.

FIG. 4 is a flow chart depicting the mid-packet detection process.

FIG. 5 is an example of a plot of a receive signal that is analyzedusing the mid-packet detection process.

FIG. 6 is an example of a plot of a first autocorrelation signalproduced by the mid-packet detection process from the receive signalshown in FIG. 5.

FIG. 7 is an example of a plot of a peak matched filtered signalproduced by the mid-packet detection process from the firstautocorrelation signal shown in FIG. 6.

FIG. 8 is an example of a plot of a second autocorrelation signalproduced by the mid-packet detection process from the peak matchedfiltered signal shown in FIG. 7.

FIG. 9 is an example of a plot of an average signal produced from thesecond autocorrelation signal shown in FIG. 8.

FIG. 10 is an example of a plot of a normalized average signal producedfrom the average signal shown in FIG. 9.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

Techniques are provided herein for a packet detection scheme that isuseful when the preamble or other start-of-packet (SOP) pattern or fieldis not observed. According to the technique, energy is received in afrequency band and a receive signal is produced that represents thereceived energy. The receive signal is analyzed to detect a packetassociated with a transmission by a communication device at a point intime other than a preamble pattern that is associated with the packet.

Example Embodiments

Referring first to FIG. 1, an example of a packet or frame is shown at10. The packet 10 is one example of a type of packet whose presence isto be detected using a mid-packet detection process described herein,and FIG. 1 is meant by way of example only. The packet 10 comprises apreamble 20 and a payload 30. The payload 30 further comprises aplurality of segments 40(0)-40(N-1) that occur at regular intervalsthroughout a duration of the packet. For example, each segment is asymbol, such as an orthogonal frequency division multiplexed (OFDM)symbol. Thus, segment 40(0) contains symbol 0, segment 40(1) containssymbol 1 and so on. Each of the segments 40(0)-40(N-1) comprises a datainterval 50 and a guard interval (GI) 60. The segments occur at aregular interval referred to herein as a repetition period. That is, theduration of a segment (symbol) is the repetition period of the segments.While FIG. 1 shows that the GI 60 is at the start (also known as acyclic prefix) of a segment, it may occur at the end (also known as acyclic postfix) of a segment. The GI 60 is provided to avoidinter-symbol interference and thereby reduce error rate caused bymultipath effects. For example, in the IEEE 802.11n communicationstandard, the duration of the GI is 800 ns (referred to as a “long” GI)or 400 ns (referred to as a “short” GI) and appears at the start of eachOFDM symbol transmitted in the corresponding packet and thereforepossesses a desirable property that can be used to detect OFDM symboltransmissions indicative of a packet.

A packet containing OFDM symbols is one example of a packet for whichthe mid-packet detection technique is useful as described herein. AnOFDM symbol appears as Gaussian noise yet it can be identified by itsregular cyclic extension. As mentioned above, a cyclic prefix is arepeat of a portion (e.g., the start) of the data part of an OFDMsymbol. The GI allows multipath effects of the channel to settle beforeeach data portion of an OFDM symbol arrives at the receiver. Thus, themid-packet detection process described herein uses the cyclic prefixextension of a symbol, e.g., an OFDM symbol, in order detect a packet ata point in time other than occurrence of the preamble of the packet.This generally corresponds to the non-preamble packet detection timeinterval shown at reference numeral 70 in FIG. 1. More specifically, themid-packet detection process analyzes the receive signal on the basis ofthe repetition period of the segments (symbols) and duration of the GIof the segments (symbols) in a packet of a type expected to betransmitted in a frequency band of interest.

Although the terms “OFDM symbol” and “prefix” are used herein, it is byway of example only and without any intention to limit the scope of thetechniques described herein to OFDM symbols or prefix techniques. Thetechniques described herein may be used to detect any type of waveformthat has portions with fixed durations and that periodically repeatthroughout the duration of the waveform.

FIG. 2 shows an example of a communication device 100 configured toperform the mid-packet detection process. The communication device 100comprises two antennas 110(1) and 110(2), a downconverter 120, anupconverter 130, a modem 140 and a controller 150. The downconverter 120is configured to produce receive signals from energy detected by antenna110(1) and 110(2), respectively. In other embodiments, the communicationdevice 100 may have 3, 4 or more antennas. The upconverter 130 isconfigured to make transmissions via one or both of the antennas 110(1)and 110(2) based on a transmit signal supplied to it by the modem 140.The modem 140 comprises mid-packet detection logic (e.g., hardware) 200that performs the mid-packet detection process described herein. Thecontroller 150 performs overall control functions of the communicationdevice 100. It is to be understood that the downconverter 120 mayactually comprise two separate downconverter circuit paths, one for eachof the antennas 110(1) and 110(2), but for simplicity, these individualdownconverter circuit paths are not shown in FIG. 2. In otherembodiments, the communication device 100 may have 3, 4 or moredownconverter circuit paths according to the number of antennas.Likewise, the upconverter 130 may comprises separate upconverter circuitpaths, one for each of the antennas 110(1) and 110(2), but forsimplicity these individual upconverter circuit paths are not shown inFIG. 2. The communication device 100 further comprises an interfaceblock 160 that is configured to interface signals from the device 100 toother devices or systems and from other devices or systems. For example,the interface block 160 may be component that implements the peripheralcomponent interface (PCI) standard.

As indicated above, the mid-packet detection technique described hereindetects a packet at a point in time of the packet other than a preamblethat is associated with the packet and indicates the beginning of thepacket. Packet detection without detecting a preamble should not requirecarrier frequency recovery, timing recovery (synchronization) or channelestimation, since these are much more difficult signal processing tasksto perform in the absence of a preamble. The method, apparatus and logicdescribed herein does not require carrier frequency recovery, timingrecovery (synchronization) or channel estimation, and relies onlyregularly repeating segments within the packet waveform. In oneembodiment, a doubly differential matched filter autocorrelation of thereceived signal is computed and used as a metric for packet detectionwhen the preamble is not observed or to complement preamble detection.The metric is compared to a threshold, which when exceeded, indicatesdetection of a packet.

An example of the mid-packet detection logic 200 is now described withreference to FIG. 3. The mid-packet detection logic 200 comprises afirst autocorrelation block 210, a matched filter block 220, a secondautocorrelation block 230, an averaging block 240 and a normalizing andthreshold comparison block 250. The output of the mid-packet detectionlogic 200 is at the normalizing and threshold comparison block 250. Theoutput of conventional preamble-based detection logic 300 may besupplied together with the output of the mid-packet detection logic 200to an OR gate block 310. The OR gate block 310 outputs a positive packetdetection indicating that the channel is not clear for transmission, inother words a negative clear channel assessment (CCA) indication, wheneither the mid-packet detection logic 200 or the conventionalpreamble-based detection logic 300 generates an output indicatingdetection of a packet.

The first autocorrelation block 210 comprises delay and conjugatecomputation elements 212(1) and 212(2) and multiplier elements 214(1)and 214(2) for receive signals associated with antennas 110(1) and110(2), respectively. An adder 216 is provided that adds the outputsfrom the multipliers 214(1) and 214(2). The delay and conjugatecomputation elements 212(1) and 212(2) delay the receive signalsassociated with antennas 110(1) and 110(2), respectively, by a delayamount corresponding to the duration of the data interval of a segmente.g., OFDM symbol (FIG. 1), and computes the conjugate of the delayedsignal. The multiplier elements 214(1) and 214(2) multiply the originalreceive signals at antennas 110(1) and 110(2) by the delayed andconjugated signals from elements 212(1) and 212(2), which are thensupplied to the adder 216 that combines the two signals. Thus, the firstautocorrelation block 210 comprises two autocorrelation modules, one foreach of the receive signals associated with antennas 110(1) and 110(2),produces autocorrelation signals associated with each antenna path,combines the two autocorrelation signals and produces a firstautocorrelation (vector) signal. In general, the number ofautocorrelation signals produced in the first autocorrelation block 110(to ultimately produce the first autocorrelation signal) corresponds tothe number of downconverter circuits in the communication device.

The matched filter block 220 comprises a moving average computationmodule 222 that is configured to compute a moving average of the firstautocorrelation signal, thereby serving as a matched filter of thevarious peaks in the first autocorrelation vector signal. In oneembodiment, the moving average computation module is a matched filterwith a rectangular impulse response of duration equal to the GIduration. The output of the matched filter block 220 is referred toherein as a peak matched filtered signal.

The second autocorrelation block 230 comprises a delay and conjugatecomputation element 232 and a multiplier element 234 that, incombination, serve as an autocorrelation module to compute thecorrelation between the original autocorrelation, that is, the peakmatched filtered signal and a delayed version of the peak matched filtersignal produced by the delay and conjugate computation element 232. Thedelay equals the duration of a segment (FIG. 1), e.g., an OFDM symbolduration. The output of the second autocorrelation block 230 is referredto as a second (doubly differential) autocorrelation signal.

The averaging block 240 computes averages at each downconverter sampletime of the second (doubly differential) autocorrelation signal taken atOFDM symbol periods over several OFDM symbol periods and produces anaverage signal. Said another way, the averaging block 240 computes anaverage of the second autocorrelation signal over a plurality of segmentintervals to output an average signal comprising peaks that regularlyoccur at a time interval corresponding to the repetition period of thesegments. Though there is only one peak per repetition period in theaveraged signal, the average signal is like a moving average. Theaverage signal is continuously updated with the incoming receivedsignal. As more and more symbols are contained in the received signalover time, the peaks are averaged over a sliding window consisting of4-5 symbols, for example. As a result, the output average signalcomprises peaks that regularly occur at time intervals corresponding tothe repetition period.

The normalizing and threshold comparison block 250 normalizes theaverage signal to account for amplitude variations of the average signaldue to different packet transmissions, imperfect automatic gain control(AGC), noise, etc. For example, the average signal may be normalized bycomputing the root mean squared (RMS) average of all averages computedby the averaging block 240 over the previous OFDM symbol period and thendividing the average signal by the RMS average to produce a normalizedaverage signal. Then, block 250 compares the maximum, e.g., peak, of areal part of the normalized average signal to a threshold to determinewhether a packet is present on the channel. The threshold is, forexample, a threshold in dBm. When the maximum or peak of the normalizedaverage signal exceeds the threshold, the block 250 generates an outputto indicate presence of a packet, such as a high (logic “1”) output.

More generally, the normalizing and threshold comparison bock 250compares a magnitude of peaks in the average signal (produced by theaveraging block 240) with a threshold and generates an output indicatingdetection of a packet when the magnitude of any one of the peaks exceedsthe threshold.

A mid-packet detection process is now described in conjunction with FIG.4 and FIGS. 5-10. The mid-packet detection process, shown generally at400 in FIG. 4, represents one example of the operation of the logic 200shown in FIG. 3 and described above. FIGS. 5-10 illustrate plots ofsignals at the various stages of the mid-packet detection process 400,and corresponding to the signals referred to above in the description ofFIG. 3.

At 410, energy is received in the frequency band (e.g., channel) ofinterest and a receive signal is produced that represents the receivedor detected energy. This is a function performed by the receiver 120.FIG. 5 illustrates an example of a plot of the receive signal, and inparticular a signal that has been transmitted in accordance with theIEEE 802.11n communication standard.

Next at 420, the receive signal is delayed by an amount equal to theduration of the data interval of an segment (e.g., OFDM symbol) of atype of packet expected to occur in the frequency band of interest, andan autocorrelation of the delayed receive signal is computed to producea first autocorrelation signal. In the example of the mid-packetdetection logic shown in FIG. 3, this function is performed for thereceive signal associated with each of the antenna paths. An example ofa plot of the first (singly differential) autocorrelation signalcomputed from the receive signal shown in FIG. 5 is shown in FIG. 6. Thefirst (singly differential) autocorrelation signal can contain numerouspeaks due to the nature of the modulated waveform (e.g. an OFDM symbolis Gaussian-like due to its many subcarriers).

At 430, a moving average of the first autocorrelation signal is computedto match filter the peaks in the first autocorrelation signal that arisefrom the duration of the GIs in a packet, thereby producing a peakmatched filtered signal. The moving average function gathers thenumerous peaks that the first autocorrelation signal may contain. FIG. 7illustrates an example of a plot of a peak matched filtered signal thatis computed for the first autocorrelation signal shown in FIG. 6.

At 440, an autocorrelation of the matched filtered signal is computed ata delay corresponding to the repetition period of the segments (symbols)to produce a second (doubly differential) autocorrelation signal. In theideal case (noiseless, perfect AGC), the maximum value of the firstautocorrelation should be purely real in the presence of a packet.However, carrier frequency offsets of transmitted packets candrastically affect this peak because of considerable phase changesinduced onto the signal. The second autocorrelation signal compensatesfor the effects of carrier frequency offsets of transmitted packets.FIG. 8 illustrates an example of a plot of the second autocorrelationsignal derived from the peak matched filtered signal shown in FIG. 7.The larger peaks shown in FIG. 8 correspond to individual OFDM symbolsin the receive signal. The occurrence of these peaks at a regularspacing (time interval between peaks) is indicative of the occurrence ofOFDM symbols in the receive signal. In fact, the spacing between peakscorresponds to an OFDM symbol period

At 450, an average of the second autocorrelation signal is computed atOFDM-symbol-spaced points over multiple OFDM symbol intervals to producean average signal that contains a single pulse when the secondautocorrelation signal comprises peaks that regularly occur at a timeinterval corresponding to a duration of a packet. For example, theaverage may be computed over 4-8 packet intervals. FIG. 9 shows anexample of an average signal derived from the second autocorrelationsignal shown in FIG. 8. A single relatively strong peak in the averagesignal is shown at reference numeral 452 in FIG. 9. At this point,detection of an in-progress packet can be made by comparing a magnitudeof any pulse in the average signal with a threshold and generating anoutput indicating detection (presence) of a packet in the receive signalwhen the strength of the any pulse in the average signal exceeds thethreshold. However, to account for noise and imperfect AGC, multiplepackets transmitted with at least some partial overlap in time, etc.,the average signal can be normalized before compared with a threshold.Again, said another way, the averaging function at 450 involvescomputing an average of the second autocorrelation signal at pointsspaced by a period of time corresponding to the repetition period andover a plurality of segment intervals to output an average signalcomprising peaks that regularly occur at a time interval correspondingto the repetition period.

At 460, the average signal is normalized to produce a normalized averagesignal. The real part of the normalized average signal is compared to athreshold and if it exceeds the threshold, an output is generated thatindicates a packet is present in the received signal. FIG. 10illustrates a plot as an example of a normalized average signal for theaverage signal shown in FIG. 9. The peak shown at 462 in FIG. 10corresponds to the peak 452 in FIG. 9, but normalized. A single strongpeak in the normalized average signal is indicative of the presence of apacket in the receive signal.

The mid-packet detection technique described herein is flexible todetect packets having different GI durations (e.g. long or short asdefined in IEEE 802.11n) by deploying different instances of thehardware shown in FIG. 3 with different delays and averaging periods(and impulse responses) tuned to each set of OFDM symbol and GIdurations. The average over the number of packet (symbol) periods can beset as a variable parameter depending in the particular application. Inaddition, the threshold for packet detection can be set based onobserved, theoretical and simulation results to make the technique morerobust.

While the foregoing description and accompanying drawings refer to ahardware implementation of the mid-packet detection techniques, it maybe implemented in software stored in a readable memory, and that isexecuted by a processor (e.g., controller 150 shown in FIG. 2) in acommunication device.

There are a number of situations where the ability to detect in-progresspackets or frames (i.e., beyond the time interval during which thepreamble is transmitted) is important. The following are examples ofthese situations.

Radar detection. In certain frequency bands, radar systems and devicesare given priority as to usage of the frequency band over other devices.Thus, when a radar system is detected, the other non-radar users of thatfrequency band refrain from transmitting for a certain period of time.One problem that some communication devices have is false detection ofradar systems. Often this false detection is due to collision (in timeand frequency) of multiple packets. The preambles of the multiplepackets are lost, but the energy in the colliding packets has a strongresemblance to criteria used for detecting radar pulses, thereby causingthe communication device to falsely detect the overlap of multiplepackets as a radar system. This in turn causes the communication deviceto unnecessarily switch to a different channel. Switching channelscauses delays that can drastically affect the performance (speed, time,and power) of the communication device. Thus, the techniques describedherein can analyze a receive signal to distinguish occurrence of atleast one packet (e.g., packet collisions) from occurrence of pulses ofradar transmissions in a frequency band, and thus reduce the incidenceof false radar detection in communication devices.

Legacy coexistence. Communication standards evolve over time to improveperformance and offer new features. In so doing, it is necessary toaccount for the operation of legacy devices that operate in accordancewith earlier or older versions of the communication standard. Forexample, devices configured to operate in accordance with the IEEE802.11n communication standard may operate in a wider bandwidth mode,e.g., 40 MHz, whereas devices that operate in accordance with earlierversions of the IEEE 802.11 standard operate in a standard bandwidthmode, 20 MHz, which can overlap with the bandwidth of a device in thewider bandwidth mode. The devices that have the wider bandwidth modecapability need to check for legacy devices. These situations can arisewhen a legacy device is in the middle of a packet transmission in whichcase the wider bandwidth mode device will not detect the preambleassociated with that packet at the time it looks for activity on thechannel since the preamble has already been transmitted (i.e., it ismid-packet). Without detecting the preamble, the wider bandwidth modedevice would fail to detect the legacy device and continue with itstransmission in its wider bandwidth mode which would overlap with thechannel in which the legacy device is operating. As a result, there willbe numerous packet collisions that greatly degrade the throughput of thedevices operating on that channel. The mid-packet detection techniquesdescribed herein will allow for a more precise and robust clear channelassessment (CCA) algorithm to better allow for coexistence of devices inthe scenario described above. For example, the mid-packet detectiontechniques may detect an in-progress packet transmitted by acommunication device in a particular frequency channel within afrequency band of interest so that the communication device refrainsfrom making a transmission in the particular frequency channel. Thelogic to control the communication device to refrain from making thetransmission may reside in the modem 140 or controller 150 as shown inFIG. 2. In the case where the communication device has widebandcommunication capabilities (i.e., a transmission within multiplefrequency channels combined together), the communication device may beconfigured (via logic in its modem or controller) to refrain from makinga transmission when it detects a packet in a particular frequencychannel that is one of the multiple frequency channels that it uses forwideband transmissions.

Power savings for low-power devices. Many low-power communicationdevices go into a sleep mode to conserve power. When the device goesactive again to make transmissions on a channel, other devices might bein the middle of their packet transmission. The mid-packet detectionscheme described herein can be used to detect in-progress packets sothat the device can conserve power by waiting until the channel is clear(packet transmissions have been completed) rather than transmitting thepacket while a packet has already begun transmission by another device,losing the packet, and consequently re-transmitting the packet due tothe collision with the packet transmission in-progress from anotherdevice. Thus, the controller 150 of a communication device may beconfigured to defer transmission after the communication wakes up anddetects an in-progress packet.

Dual-mode devices. Some communication devices are equipped to operateusing multiple communication standards, for example, IEEE 802.11 and theBluetooth™ communication standard. The mid-packet detection techniquedescribed herein can be used to optimize near-simultaneous packettransfers to/from a dual mode IEEE 802.11 and Bluetooth device. Inparticular, the mid-packet detection technique can assist the device inthe IEEE 802.11 mode in detecting in-progress packets and relearning theidle or busy state of the channel after an absence due to transmittingor receiving a Bluetooth packet.

Furthermore, the mid-packet detection techniques may be used to enhancethe performance of SOP detection, e.g., preamble detection, when used inconjunction with existing SOP detection methods, by serving as aconfidence measure for current SOP detection schemes.

Although the apparatus, system, and method are illustrated and describedherein as embodied in one or more specific examples, it is neverthelessnot intended to be limited to the details shown, since variousmodifications and structural changes may be made therein withoutdeparting from the scope of the apparatus, system, and method and withinthe scope and range of equivalents of the claims. Accordingly, it isappropriate that the appended claims be construed broadly and in amanner consistent with the scope of the apparatus, system, and method,as set forth in the following claims.

What is claimed is:
 1. A method comprising: receiving energy in afrequency band and producing a receive signal representative thereof;analyzing the receive signal to detect occurrence of a packet associatedwith a transmission at a point in time during a payload portion of thepacket, wherein the payload portion of the packet comprises a pluralityof segments that occur at regular intervals throughout a duration of thepayload portion and each of the segments comprises a first portion and asecond portion that contains and repeats part of the first portion, andwherein analyzing is based on a repetition period of the segments and aduration of the second portion of the segments of the packet; anddetecting an in-progress packet transmitted in a frequency channelwithin the frequency band based on said analyzing.
 2. The method ofclaim 1, wherein analyzing comprises computing an autocorrelation metricof the receive signal and comparing the autocorrelation metric with athreshold, which when exceeded, indicates presence of a packet in thereceive signal.
 3. The method of claim 1, wherein analyzing furthercomprises: delaying the receive signal to produce a delayed receivesignal; computing an autocorrelation of the delayed receive signal toproduce a first autocorrelation signal; computing a moving average ofthe first autocorrelation signal to match filter peaks that may becontained in the first autocorrelation signal and producing a peakmatched filtered signal representative thereof; and computing anautocorrelation of the peak matched filtered signal at a delaycorresponding to the repetition period of the segments to produce asecond autocorrelation signal.
 4. The method of claim 3, whereinanalyzing further comprises: computing an average of the secondautocorrelation signal at points spaced by a period of time and over aplurality of segment intervals to output an average signal comprisingpeaks that regularly occur over time; and comparing a magnitude of thepeaks in the average signal with a threshold and generating an outputindicating detection of a packet when the magnitude of any one of thepeaks exceeds the threshold.
 5. The method of claim 4, and furthercomprising normalizing the average signal to produce a normalizedaverage signal, and wherein comparing comprises comparing a real part ofthe normalized average signal with a threshold.
 6. The method of claim1, and further comprising refraining from making a transmission in theparticular frequency channel based on the detecting.
 7. The method ofclaim 1, wherein the first portion of each segment is a symbol and thesecond portion of each segment is a guard interval.
 8. An apparatuscomprising: a receiver configured to produce a receive signal fromdetected energy in a frequency band; and a modem coupled to thereceiver, wherein the modem is configured to analyze the receive signalto detect a packet associated with a transmission at a point in timeduring a payload portion of the packet, wherein the payload portion ofthe packet comprises a plurality of segments that occur at regularintervals throughout a duration of the packet and each of the segmentscomprises a first portion and a second portion that contains and repeatspart of the first portion, wherein the modem is configured to analyzethe receive signal based on a repetition period of the segments and aduration of the second portion of the segments of the packet to detectan in-progress packet transmitted in a frequency channel within thefrequency band.
 9. The apparatus of claim 8, wherein the modem isconfigured to analyze the receive signal by computing an autocorrelationmetric of the receive signal and comparing the metric to a threshold,which when exceeded, indicates presence of a packet in the receivesignal.
 10. The apparatus of claim 8, wherein the modem is configured toanalyze the receive signal by: delaying the receive signal to produce adelayed receive signal; computing an autocorrelation of the delayedreceive signal to produce a first autocorrelation signal; computing amoving average of the first autocorrelation signal to match filter peaksthat may be contained in the first autocorrelation signal and produce apeak matched filtered signal representative thereof; and computing anautocorrelation of the peak matched filtered signal at a delaycorresponding to the repetition period of the segments to produce asecond autocorrelation signal.
 11. The apparatus of claim 10, whereinthe modem is further configured to analyze the receive signal by:computing an average of the second autocorrelation signal at pointsspaced by a period of time and over a plurality of segment intervals tooutput an average signal comprising peaks that regularly occur overtime; and comparing a magnitude of the peaks in the average signal witha threshold and generating an output indicating detection of a packetwhen the magnitude of any one of the peaks exceeds the threshold. 12.The apparatus of claim 11, wherein the modem is further configured tonormalize the average signal to produce a normalized average signal andto compare a real part of the normalized average signal with athreshold.
 13. The apparatus of claim 8, wherein the modem is furtheredto analyze the receive signal to distinguish occurrence of at least onepacket from occurrence of pulses associated with radar transmissions inthe frequency band.
 14. The apparatus of claim 8, wherein the firstportion of each segment is a symbol and the second portion of eachsegment is a guard interval.
 15. A non-transitory computer readablemedia storing software instructions for execution by a processor, andwhen executed operable to: analyze a receive signal derived from energydetected in a frequency band to detect occurrence of a packet associatedwith a transmission at a point in time during a payload portion of thepacket, wherein the payload portion of the packet comprises a pluralityof segments that occur at regular intervals throughout a duration of thepayload portion and each of the segments comprises a first portion and asecond portion that contains and repeats part of the first portion, andwherein the instructions operable to analyze are based on a repetitionperiod of the segments and a duration of the second portion of thesegments of the packet; and detect an in-progress packet transmitted ina frequency channel within the frequency band based on the analysis. 16.The computer readable media of claim 15, wherein the instructionsoperable to analyze comprise instructions operable to compute anautocorrelation metric of the receive signal and compare the metric to athreshold, which when exceeded, indicates presence of a packet in thereceive signal.
 17. The computer readable media of claim 16, wherein theinstructions operable to analyze comprise instructions operable to:delay the receive signal to produce a delayed receive signal; compute anautocorrelation of the delayed receive signal to produce a firstautocorrelation signal; compute a moving average of the firstautocorrelation signal to match filter peaks that may be contained inthe first autocorrelation signal and produce a peak matched filteredsignal representative thereof; and compute an autocorrelation of thepeak matched filtered signal at a delay corresponding to the repetitionperiod of the segments to produce a second autocorrelation signal. 18.The computer readable media of claim 16, wherein the instructionsoperable to analyze comprise instructions operable to: compute anaverage of the second autocorrelation signal at points spaced by aperiod of time and over a plurality of segment intervals to output anaverage signal comprising peaks that regularly occur over time; andcompare a magnitude of the peaks in the average signal with a thresholdand generating an output indicating detection of a packet when themagnitude of any one of the peaks exceeds the threshold.
 19. Thecomputer readable media of claim 16, wherein the instructions operableto analyze comprise instructions operable to normalize the averagesignal to produce a normalized average signal and to compare a real partof the normalized average signal with a threshold.
 20. The computerreadable media of claim 16, wherein the first portion of each segment isa symbol and the second portion of each segment is a guard interval.